It is known that images of certain physical properties of underground formations can be produced as images developed over the wall of the borehole, i.e. the cylindrical surface is presented as a two-dimensional “flat” image. In order to produce such images, it has been necessary to make measurements that are resolved in both depth and azimuth on the surface of the borehole. These measurements can be used to create image pixels that present the different values of the measured property as a variation in color or shade.
There are various properties which can be measured and various ways in which such measurements can be made in order to provide the depth and azimuthally resolved measurements needed to construct the image. For example, where the measured property of the formation is its electrical resistivity, measurements can be made using pads carrying button electrodes in an array that ensure that overlapping portions of the borehole are covered as the tool is logged up the well. An example of this is the FMI tool of Schlumberger (see U.S. Pat. No. 4,4468,623 and U.S. Pat. No. 4,567,759). Another property that has been measured is the acoustic impedance, which can be done by scanning an ultrasonic beam circumferentially in the well as the tool is logged up the well. An example of this is the UBI tool of Schlumberger (see U.S. Pat. No. 4,255,798). There are other properties that can be measured and techniques for measuring them that results in the same set of depth and azimuthally resolved data.
There are certain properties that to date cannot be determined in terms of azimuth, or certain tools that cannot provide azimuthal resolution of measurements. Such measurements give a single value for that property at a given depth in the well. This results in a classic log output with a varying magnitude of the property in question plotted against depth in the well. Examples of such depth-only measurements are the measurement of density or of photoelectric factor of the formation, of permeability as obtained from a nuclear magnetic resonance measuring device, or dielectric constant or wave attenuation, as recorded by means of an electromagnetic propagation measuring tool. In each case, measuring devices are mounted on a tool having pads that are applied to the wall of the borehole in a given direction, and they therefore deliver for each depth increment, a value for the physical magnitude in question, as measured only in that direction. Such measurements suffer inherently from lack of coverage as a function of azimuth, which constitutes a limitation in formations that are heterogeneous, e.g. of the nodular, lenticular, conglomerated, fractured, or crossbedded type.
Other properties of formations that are typically only presented in depth-resolved form are qualitative indicators of formation type, such as lithofacies. Such properties or indicators are often derived from analysis of one or more measured properties and can be used to assist in interpretation of the formation structure and properties.
“Near-wellbore 3D reconstruction of sedimentary bodies from borehole electrical image” published by H. Anxionnaz and J. P. Delhomme, Transactions of the 38th annual symposium of the SPWLA (1998), discloses a method of reconstructing a physical magnitude three-dimensionally in the surroundings of a borehole from a two-dimensional image of formation resistivity as developed over the wall of the borehole obtained using an FMI type tool which produces an image having resolution of centimeter order.
It is an object of the present invention to provide a method that allows the production of images in two dimensions or above of formation properties that are normally measured only as a function of depth in a borehole.